Battery-powered portable or implantable biopotential and bioimpedance measurement devices are becoming increasingly widespread in the medical diagnostics field. The signal acquisitions of the main biosignal-sensing applications such as electroencephalography (EEG) and electrocardiography (ECG) involve voltage measurements from a few microvolts to several millivolts [1]-[3]. Biopotentials are conventionally acquired using electrodes covered with electrolyte gels or solutions to decrease the contact impedance at the skin interface to values below 10 KΩ. However, wet-contact measurements cause discomfort and dry out in novel long-term monitoring applications such as in brain-computer interfaces where EEG signals are acquired and analyzed over hours or longer [4].
In general, dry electrodes such as inexpensive Ag/AgCl are better suited for long-term monitoring, but their use is associated with increased contact resistances that can be above 1 MΩ [5]. This characteristic complicates the measurement of small biopotentials in the range of few μV for EEG applications by requiring very high input impedance at the analog front-end amplifier of at least 500 MΩ [6]. Nevertheless, a significant problem is that this impedance is affected by parasitic capacitances of the integrated circuit package as well as electrode cable and printed circuit board (PCB) capacitances that could be as high as 50-200 pF at the input of an instrumentation amplifier (IA) shown in FIG. 1. For instance, when the goal is to record EEG signals with frequencies up to 100 Hz, an interface capacitance of 200 pF would limit the input impedance at 100 Hz to approximately 8 MΩ, which is much less than 500 MΩ and would cause excessive attenuation such that the EEG signal cannot be measured reliably.